you are right. It is the original Mustang prototype that Iococca canned. Year is 1963. This was a mid-engined moncoque bodied sports car with adjustable steeringwheel and pedals, not the girlie thing Iococca finally got.
This design study did go on to become the GT40 which was develped out of house, along with the twin cam V8 for it, also out of house but both in Dearborn.
The next Mustang was also mostly developed from the design mockups by the same Dearborn Ford dealer who later built the GT40. It was in-line 6. Final performance development was done in California by Bill(?) Miller who also entered it at Sebring, drove there from San Diego with the Mustang, raced, won the class and drove it back to California. Miller wrote my father, that, "it would blow Shelby's Cobra (then the 289) into the weeds!" This was the year before the Mustang was officially announced. This one was also canned by Iococca because, "the dealers want a V8."
That is how the myth that Iococca created the Mustang came to be. All this can be researched in my father's correspondence and papers
at the Automotive History Museum in Detroit where I gave it after his death. I kept the original 4x5 transparence shown above.